LONDON, Oct 5 (Reuters) - Teva’s blockbuster multiple sclerosis treatment Copaxone will face additional generic competition in Europe, just two days after Mylan NV won U.S. approval to sell cheaper versions of the medicine in the world’s largest market.

Mylan’s European partner, Synthon, and Alvogen said on Thursday they had received decentralized European approval for a 40 milligram dose of glatiramer acetate, as Copaxone is known generically.

The two privately held companies, which work as partners in selling the product, already market a 20 mg version in Europe. However, the 40 mg dose accounts for more than 85 percent of U.S. prescriptions written and more than 75 percent of European prescriptions.

Nearly 20 percent of total sales for Teva Pharmaceutical Industries comes from branded Copaxone. The widely used injectable MS treatment generated more than $4 billion in revenue for the Israeli drugmaker last year.

Teva’s shares were hit hard following U.S. approval of both 20 mg and 40 mg generics for Mylan late on Tuesday. Mylan shares rose about 18 percent the next day.

Mylan Chief Executive Heather Bresch, in a statement, said the company looks forward to “leading the marketing and selling of this important product in our European markets.”

Alvogen said Europe accounted for more than 505 million euros ($591 million) of Copaxone sales in 2016, based on IMS Midas data.

Generic competition to Copaxone adds to problems at Teva, which hired a new chief executive last month as it struggles with huge debts and a squeeze on margins for its own vast portfolio of generic medicines. ($1 = 0.8540 euros) (Reporting by Ben Hirschler in London and Bill Berkrot in New York; Editing by Jane Merriman and James Dalgleish)